The present invention relates to a plasma generator.
It is known that surface working operations of objects and materials (such as silicon wafers, metal items, etc.) can be performed through use of plasma, i.e. a gas on which striking of a suitable electromagnetic radiation has been caused, so as to ionise the gas itself; said plasma is generally referred to as “ionised gas”.
The radiation used can be a radiofrequency (RF) radiation or microwaves.
The gas to be ionised and the electromagnetic radiation are caused to interact within a suitably insulated chamber, in which microwaves are introduced by means of a waveguide; for passage from the waveguide to the plasma generating chamber the waveguide itself is provided with one or more openings.
In more detail, a waveguide enables propagation of the microwaves from a generator to a propagation chamber, in which the microwaves propagate following a typically circular path.
Said openings are generally formed in the lower surface (or base surface) of the propagation chamber, at such a path.
The plasma generating chamber is separated from the propagation chamber exactly by the lower surface of said propagation chamber.
The openings in the propagation chamber are usually identical with each other and can have several different shapes and arrangements. For instance, the European Patent EP0880164 discloses slots of elongated shape substantially identical with each other and disposed in radial directions on the separation plate between the waveguide and the plasma generating chamber.
However, it is to be pointed out that microwaves reduce their intensity as they move away from their source (i.e. the generator connected upstream of the waveguide), and in particular they reduce their intensity as they propagate from the waveguide outlet to the inside of the propagation chamber.
Therefore, the microwave ionisation capability progressively decreases as the microwaves move away from the intersection between the waveguide and the propagation chamber.
On the contrary, the gas to be ionised is usually distributed in a uniform manner within the plasma generating chamber; this involves a non homogeneous distribution of the plasma obtained by intersection between the microwaves and the gas present in the plasma generating chamber.
In fact, at the waveguide outlet a greater amount of plasma will be obtained (because the microwave intensity is greater), while at the farthest point (diametrically opposite, in the event of a cylindrical structure) the ionised-gas amount will be much smaller, due to the lower intensity of the microwaves.
As a result, plasma distribution in the generating chamber will be non uniform, which will clearly adversely affect the quality of the surface treatment to be carried out with the plasma thus obtained.